1. Technical Field
The embodiments described herein relate to a phase change memory device, and more particularly, to a phase change memory device having a protective layer and a method for manufacturing the same.
2. Related Art
Generally, a phase change memory device is a memory device that writes and reads information by changing a phase of a phase change material from an amorphous state, which has a high resistance, to a crystalline state, which has a low resistance. The phase change memory device is advantageous over a flash memory device in that it has a fast operational speed and a high-level integration. A representative phase change material includes a chalcogenide compound that is commonly made of germanium (Ge), antimony (Sb), and tellurium (Te) constituents, which are collectively referred to as a GST chalcogenide.
Since the phase changes continuously occur from the crystalline state to the amorphous state and vice versa, the repeated volumetric expansion and contraction of the phase change material generates heat, wherein the phase change material is likely to delaminate from bottom electrode contacts (BECs). In addition, since the phase change material is formed using the composite compound described above, the constituents of the phase change material are likely to diffuse through adjacent layers during fabrication processing.
In order to prevent or avoid the delamination of the phase change material from the bottom electrode contacts and to prevent the diffusion of the constituents of the phase change material, a protective layer can be formed as an encapsulator so as to prevent degradation of the properties of the phase change material during the phase changing. For example, a silicon oxide layer and a silicon nitride layer can be used as the protective layer.
When the protective layer is formed of a silicon oxide layer, can be difficult to prevent the diffusion of the constituents of the phase change material into the protective layer, and the silicon oxide layer may be recombined with the diffusing constituents to create an interface having abnormal composition. This abnormal composition interface can adversely influence the operational characteristics of the phase change material and can actually facilitate the diffusion of the constituents of the phase change material to further degrade the properties of the phase change material.
Meanwhile, when the protective layer is formed of a silicon nitride layer, since the silicon nitride layer is formed at a substantially high temperature over 400° C., a thermal burden can be imposed on the phase change material.
In addition, because the silicon nitride layer has poor step coverage characteristics, it cannot be sufficiently deposited to a substantially uniform thickness on the sidewalls of a phase change material layer. As a consequence, the silicon nitride layer can be formed in the shape of relatively thick overhangs on the upper edges of the phase change material layer, and can be formed to have relatively thin portions on lower portions of sidewalls of the phase change material layer.
Accordingly, the presence of the overhangs makes it difficult to properly fill spaces formed between adjacent phase change material layers using a buried insulating layer. Furthermore, when forming the buried insulating layer using a high density plasma oxide layer, the relatively thin portions of the silicon nitride layer are likely to be damaged by the applied plasma used for forming the high density plasma oxide layer. Thus, corresponding portions of the phase change material can become exposed, thereby changing the properties of the phase change material and adversely influencing the driving of the phase change memory device.